This invention relates to improvements in intruder detection systems of the type which sense the presence of an intruder within the boundaries of a space under surveillance by sensing an abnormal rate of change in the ambient temperature of such space, as would be occasioned by the movement therethrough of an intruder whose body heat differs from the steady state ambient temperature.
In U.S. Pat. No. 3,839,640, issued to J. A. Rossin, there is disclosed a passive infrared intruder detection device which employs a pair of heat-sensitive capacitors as the intruder-sensing elements. Each of these capacitors comprises a small piece of polyvinylidene fluoride (PVF.sub.2) plastic film sandwiched between a pair of electrodes. This particular film, when appropriately polarized, is well known for its excellent pyroelectric properties, i.e. its ability to generate a readily detectable voltage in response to experiencing a relatively minute change in temperature.
In the Rossin device, the two pyroelectric capacitors are connected in series opposition and are positioned side-by-side at the focus of a concave front surface mirror. By virtue of their difference in position relative to the mirror, each of these detectors has a field of view which differs slightly from the other. When an intruder enters one of the fields of view of the device, his body heat causes a momentary change in the temperature of one of the detectors. This unbalance in the system is detected and an alarm signal is generated in response thereto.
While the Rossin device is functional, it is inherently a narrow field of view device because adding additional detectors, as would be required to expand the field of view, cannot be conveniently effected without complicating the structure, as well as the manufacture of the array. Also, adding additional detectors in series would cause a considerable increase in the impedance of the array, making it difficult to find a suitable preamplifier. It would also cause substantial signal loss and thereby drastically reduce the sensitivity of the device. Another inherent disadvantage of the Rossin device results from the fact that the two detectors share a common electrode which floats electrically. Since the common lead cannot be grounded, the device can be susceptible to static charge build-up and electrical pick-up which would become increasingly troublesome as the number of detectors in series is increased.
Another passive infrared intruder detection system is disclosed in U.S. Pat. No. 3,958,118 issued to F. Schwarz. Such a system includes a plurality (e.g. four) heat-sensitive capacitors which are electrically connected in parallel. While such a parallel arrangement of detectors is not subject to the aforementioned problems associated with the series-opposition connection of the detectors in the Rossin system, it is subject to false alarming as a result of sudden changes in pressure or temperature which affect two or more of the detectors simultaneously. For instance, a door slamming closed in the protected area will cause all of the detectors to generate a voltage simultaneously; this voltage results from the piezoelectric properties of the detectors. Likewise a thermal gradient will produce an output from all detectors simultaneously. Such events will produce false alarming of the system unless compensated for by relatively complex signal processing circuitry which can differentiate such events from the event of interest (i.e. intrusion).